14266 - Implementation of NGS in the MCU Process for Increased Removal of Cesium at SRS

15 Pages·2014·0.4 MB·English

WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

Implementation of Next Generation Solvent in the Caustic-Side Solvent Extraction
Process for Increased Removal of Cesium at Savannah River Site – 14266
Earl Brass *, Celia Aponte *, Edward Ketusky *, Annah Garrison **
* Savannah River Remediation, LLC
** URS - Professional Solutions, LLC

ABSTRACT

Caustic Side Solvent Extraction (CSSX) is currently used at the Department of Energy Savannah
River Site (DOE SRS) for removal of cesium from the high-level salt-wastes stored in
underground tanks. Currently, the actinide removal process (ARP) and the CSSX process are
deployed in the ARP/Modular CSSX Unit (MCU), to process salt waste for permanent disposition.
The CSSX technology utilizes a multi-component organic solvent and annular centrifugal
contactors to extract cesium from alkaline salt waste. The original plant was permitted for a three
year operating life; however, given the successful operation of the plant, a life extension program
was completed to continue operations. As part of the life extension program, a new solvent
called Next Generation Solvent (NGS), which was developed by Oak Ridge National Laboratory
(ORNL) with funding provided by EM Technology Development, will be implemented at MCU to
decrease the curies of cesium going to grout in the Saltstone Production Facility (SPF). NGS is
an engineered solvent designed to increase the extraction performance of cesium from salt waste
and with cold feed chemical changes increase the corresponding cesium stripping performance,
resulting in a salt waste effluent with substantially lower cesium concentration. Implementation
of NGS in the MCU at SRS is being performed to maintain previous commitments to the state of
South Carolina with regard to the curies of cesium going into cementitious grout being left in the
state, which could be impacted under the original CSSX flowsheet. In order to implement NGS in
MCU, a technical maturation plan (TMP) was developed to identify technical maturity level gaps
that required further evaluation, testing, or engineering work in order to bring the immature
technology to the appropriate maturity level. The TMP was utilized as the guide to promote the
Technical Readiness Level (TRL) from TRL-2 (i.e. technology concept and/or application
formulated), with the corresponding activities necessary to reach a TRL-6 (i.e. engineering/pilot
scale system in relevant environment). Critical technology maturation activities included full
scale testing, combined hazards assessments for three SRS facilities, and Project/Facility
documentation/modifications for incorporation of the NGS flow sheet.

INTRODUCTION

The SRS liquid waste system consists of multiple facilities to safely receive and store legacy
radioactive waste, treat, and permanently dispose of waste. The large underground storage
tanks and associated equipment, known as the 'tank farms', include a complex interconnected
transfer system which includes underground transfer pipelines and ancillary equipment to direct
the flow of waste. The waste in the tanks is present in three forms: supernatant, sludge, and
1
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

saltcake. The supernatant is an alkaline aqueous mixture, while sludge consists of insoluble
solids and entrapped supernatant. The saltcake results from the evaporation (water removal) of
the supernatant. The tank waste is retrieved and treated as sludge or salt solution. Saltcake is
retrieved from the waste tanks by removing the interstitial liquid, dissolving the saltcake with
water, and finally retrieving the dissolved salt solution for further processing [1]. The high level
(radioactive) fraction (actinides, strontium, and cesium) of the waste is vitrified into a glass waste
form, while the low-level waste is immobilized in cementitious grout waste called saltstone.
Once the bulk of the waste is retrieved and processed, the tanks are closed via chemical cleaning,
heel removal, stabilizing remaining residuals with tailored grout formulations and severing/sealing
external penetrations. The comprehensive liquid waste disposition system, currently managed
by Savannah River Remediation (SRR), consists of 1) safe storage and retrieval of the waste as it
is prepared for permanent disposition; (2) definition of the waste processing techniques utilized to
separate the high-level waste fraction/low-level waste fraction; (3) disposition of Low Level Waste
(LLW) in saltstone; (4) disposition of the High Level Waste (HLW) in glass; and (5) closure state of
the facilities, including tanks. This paper focuses on the technical maturation of a new solvent
(i.e., NGS) for cesium extraction from high-level salt wastes for implementation in the MCU.
MCU was originally designed and permitted with a three year operating life and a five year design
life, bridging the period before startup of the Salt Waste Processing Facility (SWPF). A life
extension evaluation [2] was performed in 2009 and concluded that passive process components
have a service life beyond year 2025. Active components designed to be replaced with existing
spares includes process pumps. Major maintenance repairs are anticipated after 2015 which
includes replacement of all contactor bearings and wiring in closed proximity to the strip effluent
stream. With the extended operation of the MCU beyond its initial operating life forecast,
previous commitments to the state of South Carolina with regard to the curies of cesium in the
cementitious grout being left in the state, could be impacted under the current solvent flowsheet
[3]. As a result, an improved flowsheet based on an improved solvent (NGS) developed by
ORNL has been developed to improve the extraction and stripping of cesium from clarified salt
solution (CSS). This new flowsheet employs a new extractant and suppressor in the solvent as
well as new scrub and strip cold chemical feeds.

BACKGROUND

SRS Salt Processing
The existing ARP/MCU process has been operating at SRS since April 2008 as an interim salt
treatment system to remove actinides and cesium from high level waste salt solutions. MCU is
the first production-scale CSSX process for cesium separation. While the application of this
solvent extraction process is unique, the process uses commercially available centrifugal
contactors for the primary unit operation as well as other common methods of physical separation
of immiscible liquids (i.e. coalescers and decanters). This process enables permanent disposal
of the CSSX raffinate, Decontaminated Salt Solution (DSS), at the SPF as cementitious grout,
2
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

and the Strip Effluent (SE) containing cesium at the DWPF in a vitrified waste form. MCU is
operated in series with ARP, where ARP first removes sludge solids, soluble strontium and
actinides from the salt waste solution and then feeds the CSS to MCU for cesium extraction.
Figure 1 provides an overview of the SRS Salt Processing Facilities that feed DWPF & SPF.

Figure 1. SRS Salt Processing Facilities
MCU Process
MCU uses a four-part organic solvent developed by ORNL: 1) diluent/fluidic carrier; 2) calixarene
extractant; 3) solubility modifier, and 4) suppressor, combined with the operation of multiple
centrifugal contactors. The solvent is used to extract cesium from the aqueous feed solution in
the extraction contactor bank consisting of seven centrifugal contactors. The DSS raffinate is
sent to a coalescer and decanter to remove any entrained solvent before transfer to Tank 50.
The cesium laden solvent from the extraction bank is scrubbed in the scrub contactor bank
consisting of two centrifugal contactors. The scrub solution removes ionic impurities in the
solvent, and also ensures that the anion pair associated with the Cs+ in the solvent is in the
optimal anion form for stripping. Cesium is then stripped from the solvent to an aqueous strip
solution in the strip contactor bank consisting of seven centrifugal contactors. This concentrated
cesium SE is then sent to a coalescer and decanter to remove solvent carried over into the strip
effluent. The cesium laden SE is sent to DWPF to be vitrified. The solvent stream exits the strip
contactor bank and is sent to the wash contactor bank consisting of two centrifugal contactors to
clean the solvent of any impurities. The solvent is then recycled into the system for further use.
As of the end of fiscal year 2013, MCU has successfully processed more than 3.9 million gallons
of salt waste utilizing the BOBCalixC6 based solvent flowsheet.

An MCU process flow schematic is shown in Figure 2.

3
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

Figure 2. MCU Process Flow Schematic

NGS IMPLEMENTATION CHEMICAL CHANGES
Solvent

NGS utilizes the new extractant MaxCalix, more formally known as
1,3-alt-25,27-Bis(3,7-dimethyloctyloxy)calix[4]arene-benzocrown-6. Like BOBCalixC6,
MaxCalix is a highly selective calixarene extractant which has a high affinity for cesium ions and a
low affinity for sodium ions. The cesium affinity for both extractants is roughly the same;
however, MaxCalix has a significantly increased solubility over BOBCalixC6 in the diluent
Isopar™ L, thus enabling the extractant concentration to be increased from 7 mM to 50 mM (i.e.,
a 7x increase).
Both solvent systems use the modifier Cs-7SB, also known as 2-Propanol,
1-(2,2,3,3-Tetrafluoropropoxy)-3-(4-sec-butylphenoxy). The modifier is designed to increase the
solubility of the extractant in the diluent to increase cesium extraction. However, with the
increased solubility of MaxCalix in the diluent, less modifier is required in the NGS system (a
reduction of 0.75 M to 0.5 M [4]). This reduction in modifier has an additional benefit of reducing
the solvent viscosity and density, which is anticipated to further improve hydraulics in the
centrifugal contactor banks and efficiency in the coalescers and decanters.
4
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

The suppressor is present in the solvent to improve stripping performance by suppressing the
effects of anion impurities and lipophilic surfactants. The BOBCalixC6 based solvent system
utilizes TOA as the suppressor. During the development of the NGS flowsheet it was determined
that TOA would not successfully function in the NGS system with the new strip chemistry. An
initial improved flowsheet was developed with DCiTG, also known as N,N'-Dicyclohexyl-N"-
isotridecylguanidine, as the suppressor in the NGS system. DCiTG is a generic version of the
active guanidine reagent in the Cognis extractant LIX™79. DCiTG was initially considered an
attractive candidate for suppressor because of its effectiveness and availability. However,
DCiTG was ultimately rejected because in testing it was found to partition too readily to the
aqueous phase [5]. Partitioning of the suppressor to the strip aqueous phase poses potential
downstream processing impacts. An alternative suppressor, TiDG, also known as
(tris(3,7-dimethyloctyl)guanidine hydrochloride), was chosen for the NGS system because of its
effectiveness, stability, and lipophilic characteristics. The working suppressor will remain the
same concentration in both solvent systems (0.003 M).
NGS and BOBCalixC6 solvent systems utilize the same isoparaffinic diluent, Isopar™ L. The
concentration of Isopar™ L will increase in the NGS system (from 69 wt. % to 74 wt. %). This
increase in diluent is also anticipated to improve hydraulics by reducing solvent density and
viscosity.
Cold Feeds

The MCU system uses scrub, strip, and wash cold chemicals in order to remove cesium and
impurities from the solvent. The BOBCalixC6 based solvent system operates under a nitrate
swing principle, which drives the transfer of ions between the aqueous and solvent phases
depending on the concentration of nitrate in the aqueous phase. The NGS system operates
under a pH-swing principle, which drives the transfer of ions between the aqueous and solvent
phases depending on the pH of the aqueous phase.

After extraction the cesium laden solvent is sent to the scrub hydraulic bank [6]. Scrubbing
removes soluble salts, such as sodium and potassium from the solvent. These salts would be
stripped in the first strip stage and negatively affect the stripping process by altering the aqueous
phase concentrations for all subsequent strip contactor stages. In the BOBCalixC6 based
solvent system the scrub solution is 0.05 M nitric acid. This ensures that the anion associated
with Cs+ in the solvent is nitrate, which promotes stripping in the BOBCalixC6 based solvent
system [7]. The NGS flowsheet changes the scrub solution to 0.025 M sodium hydroxide, which
ensures that the anion associated with Cs+ in the solvent is hydroxide, which promotes stripping in
the NGS system [8,9]. The change in the scrub solution results in the suppressor being in a
neutral form after it contacts the scrub so the suppressor can then subsequently protonate in the
strip bank. The aqueous product of the scrub contactor bank is sent back to the extraction bank,
and the solvent product is sent forward to the strip bank.

In the BOBCalixC6 based solvent system the strip solution is 0.001 M nitric acid. This low
concentration forces the migration of the concentrated cesium nitrates in the solvent to the more
dilute aqueous nitric acid [7]. However, any effort to improve the stripping of cesium in the
BOBCalixC6 based solvent system would be limited by the ability to lower the concentration of
nitrate in the strip solution. The NGS system utilizes a 0.01 M boric acid as the strip solution.
The boric acid strip was chosen for the NGS system because improved cesium stripping could be
5
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

achieved by employing a pH swing rather than a nitrate swing in either system. The boric acid
strip was determined to be more effective when nitrate ions were removed from the solvent;
hence the change from a nitrate based scrub to a hydroxide based scrub. Investigation of
alternatives suggested that replacement of the nitrate anion with a much more hydrophilic anion,
such as borate, would substantially decrease the cesium distribution coefficients associated with
the stripping [8]. Testing confirmed that replacing the nitric acid strip with boric acid would result
in increased stripping. The combination of MaxCalix as the extractant, TiDG as the suppressor,
and the use of boric acid strip provide a dramatic increase in the removal of cesium from the salt
waste. The aqueous effluent from the strip bank containing the concentrated cesium is sent
forward to DWPF for borosilicate vitrification while the stripped solvent is sent to the wash
contactor bank.

The wash is a 0.03 M sodium hydroxide solution for both solvent systems. The wash is designed
to eliminate impurities and degradation products built up in the solvent over time. This ensures
that the solvent can be effectively recycled.

Solvent Blending

The NGS Blend (mixture of BOBCalixC6 and NGS) was developed by ORNL, specifically as a
strategy [4] for introducing NGS to MCU, as it removes the hazards and risks associated with the
need to de-inventory the BOBCalixC6 based solvent out of MCU, as such an effort would be
prohibitively complex, generate significant waste, and create significant risk of potential personnel
radiological exposure. Through testing ORNL has shown the NGS Blend to be equally effective
as pure NGS.
The NGS solvent components were introduced to the MCU solvent system in a “concentrate” and
blended with the existing BOBCalixC6 based solvent to create the NGS Blend. Following
blending of the solvent, the MCU entered into a maintenance outage to replace system active
components (i.e., pumps and pumps variable frequency drives). MCU deliberate operations with
NGS will be initiated following the completion of the maintenance outage. After NGS
implementation with the NGS Blend, all solvent additions/adjustments will consist of only NGS
constituents. Therefore, the operating solvent will gradually approach the final Pure NGS
concentrations. The concentrations of the various solvents are shown in Table I.
6
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

TABLE I: MCU Solvent Concentrations
Solvent Constituent Concentrations (Nominal)
BOBCalixC6 = 0.007 M
Cs-7SB = 0.750 M
BOBCalixC6 Based Solvent
TOA = 0.003 M
Isopar-L balance (69 wt. %)
MaxCalix = 0.0465 M
Cs-7SB = 0.500 M
BOBCalixC6 = 0.0035 M
NGS Blend
TiDG = 0.003 M
TOA = 0.0015 M
Isopar-L balance (74 wt. %)
MaxCalix = 0.050 M
Cs-7SB = 0.500 M
Pure NGS
TiDG = 0.003 M
Isopar-L balance (74 wt. %)

TECHNICAL MATURATION PLAN

MCU with NGS process maturation was performed utilizing the methodologies described in the
United States Department of Energy (DOE) Technology Readiness Assessment (TRA) Guide
[10], which outlines three primary steps:
i. Establish the principal technology elements (TEs) for the proposed process, and subsequently
assess each TE with respect to its potential designation as a critical technology element
(CTE).
ii. Determine the associated technology readiness levels (TRLs) for each CTE.
iii. Ascertain potential technology gaps that might hinder technology deployment and document
strategies for closing the gaps, and maturing the technology.

The TRA provides a snapshot of the technology readiness of each CTE and demonstrates that a
TRL of 6 has been reached and the program is ready for implementation in the facility. The
various TRA levels and definitions are defined in Table II.

7
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

TABLE II: Technology Readiness Levels [10]
Relative Level of Technology
Development TRL TRL Definition
Actual system operated over the full range of
System Operations 9
expected conditions
Actual system completed & qualified though test &
8
validation
System Commissioning
Full scale, similar (prototypical) system
7
demonstrated in relevant environment
Engineering/ pilot scale, similar (prototypical)
6
system validation in relevant environment
Technology Demonstration
Lab scale similar system validation in relevant
5
environment
Technology Development
Component and/or system validation in laboratory
4
environment
Analytical and experimental critical function and/or
3
characteristic proof of concept
Research to Prove Feasibility
2 Technology concept and/or application formulated
Basic Technology Research 1 Basic principles observed and repeated

Critical Technology Elements

Two CTEs were identified by the assessment team: the contactor banks and the
coalescer/decanters [11,12]. The contactor banks, the first CTE (CTE 1), have two objectives
critical to the MCU process. The first objective is to extract cesium from CSS to produce DSS.
The second objective is to strip cesium from the solvent, so the solvent can be recirculated and
re-used. The second CTE (CTE 2) is the coalescer/decanter combination. The objective of this
technology is to sufficiently reduce organic carryover in the aqueous product streams. The
coalescer increases the size of entrained organic droplets. The decanter separates the organic
and aqueous streams. Coalescers are the key to minimizing loss of organic traces to
downstream processing, minimizing downstream flammability problems therein, and recovering
valuable solvent components.
CTE 1
Centrifugal contactors boast ease of operation, rapid attainment of steady state, high mass-transfer
rate, rapid coalescence, and compact size, making them the preferred device for solvent-extraction
processing. The contactors were manufactured by Costner Industries Nevada Corporation
(CINC). The contactors are of two sizes, a 10 inch rotor diameter (i.e., V-10), to accommodate
high flow rates, and a smaller 5-inch rotor diameter (i.e., V-05), for lower flow rates. The extraction
8
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

bank consists of seven V-10 contactors. The cesium laden solvent is then sent to the scrub bank,
which consists of two V-05 contactors. The cesium-enriched scrubbed solvent then passes into
the strip bank, consisting of seven V-05 contactors, where the cesium is removed from the solvent
phase (i.e., stripped) to the aqueous SE. Any acidic carryover from the strip bank, the solvent
degradation products, and other contaminants will be neutralized/removed from the solvent using a
dilute caustic solution wash with the two V-05 wash contactors.
CTE 2
The two aqueous streams leaving MCU, the SE and DSS streams, contain small droplets of
organic solvent which must be separated before the streams can be sent to downstream facilities.
To achieve separation each stream passes through a coalescer and decanter in series.
Prior hydraulic testing of the V-10 contactors with BOBCalixC6 based solvent and simulated salt
solution indicated that there could be up to 1,000 ppm carryover of the solvent in aqueous stream
exiting the contactor with the majority of the droplets being less than 10 µm in diameter [13]. If
the DSS stream is sent to the downstream facility with this level of organic carryover, it could lead
to flammability concerns to downstream processes as well as rapidly deplete the inventory of
solvent in MCU. To remove the flammability potential and recover the solvent, a coalescer and
decanter are installed in series downstream of the extraction contactors. Coalescers promote
the merging of many small droplets to form fewer droplets with a larger diameter. The larger
diameter increases the buoyant forces of the droplet (Stokes Law) requiring considerably less
time to decant. The DSS coalescer media used is a 40 inch long, 20 micron polyphenylene
sulfide type mesh. The DSS Decanter includes a baffle configuration to ensure the waste has
sufficient time for the Cs-137 daughter product, Ba-137m, to decay at least 10 half-lives.
Prior hydraulic testing of the V-05 contactors indicated that there could be up to 400 ppm
carryover of solvent in the aqueous SE stream exiting the contactors with the majority of the
solvent droplets being less than 10 µm in diameter [13]. If the SE stream is sent to the
downstream facility with this level of organic carryover it would potentially cause a flammability
concern in the DWPF facility, so a coalescer and decanter are installed downstream of the strip
contactors to capture excess solvent carryover. The SE coalescer media used is a 20 inch long,
10 micron polyphenylene sulfide type mesh.

Technical Maturation Plan Progression
Because MCU is an operational facility, the TRA process and TRL determination must be taken in
context. The technology is in place and the flowsheet has been verified; a new component is
simply being added to the system. The MCU with BOBCalix solvent has achieved a TRL of 9
with radiological operation. A self-conducted TMP performed for the “Pure NGS” with DCiTG as
the suppressor identified the Technical category TRL to be 3, Programmatic category TRL to be
3, and the Manufacturing/Quality category TRL to be 6 [11,12]. Since issuance of the Pure NGS
TMP, ORNL had further optimized NGS by replacing the suppressor with TiDG, and developed
the Blend strategy as a way of introducing NGS, without assuming the risks associated with first
9
WM2014 Conference, March 2 – 6, 2014, Phoenix, Arizona, USA

removing the BOBCalixC6 based solvent. Based on the change in suppressor and the strategy
to blend the solvents, the assessment team issued a revision of the TMP [14] which identified the
contactor banks CTE as an overall TRL-2 and the coalescer/decanter CTE as an overall TRL-2.
The initial readiness levels in each of the assessment categories (Technology, Manufacturing and
Quality, and Programmatic) for both CTEs after the change to TiDG and a blended solvent is
shown in Table III.
TABLE III: Initial Technical Readiness Levels for CTEs with Consideration of New Suppressor
and Blend Strategy [14]
Technology Manufacturing & Quality Programmatic
TRL
Contactor Bank (CTE 1) 2 3 2
Coalescer & Decanter TRL
2 3 2
(CTE 2)

The assessment team identified the tasks shown in Tables IV and V as being required to advance
the TRL levels of the contactor banks CTE and coalescer/decanter CTE to TRL 6.

10